Scroll machines are widely employed in various applications. Recent examples of scroll machines for fluid compression or expansion, without limitation, are addressed in recent U.S. Pat. Nos. 5,342,184, 5,368,446 and 5,370,513, hereby expressly incorporated by reference. In general, scrolls employed in scroll machines may be of a variety of different types. Examples of scroll types include, without limitation, rotating, orbiting and fixed types. Ordinarily at least two scrolls are used, in co-acting combination with each other, in a scroll machine. At least one of the scrolls is a metallic structure having intricate geometries. For instance, typical scroll structures incorporate a plurality of adjoining sections having relatively large section thickness differentials or gradients relative to each other. In service, these scrolls often times encounter strenuous working conditions, and thereby desirably employ materials that will exhibit excellent wear resistance and strengths on the order of 250 MPa or greater. In view of the complexities of shape, and taking into account other material property and processibility requirements, it has been common to manufacture scrolls by casting the scrolls with a cast iron material.
The use of presently available casting materials has presented limitations in improving the design of scrolls and in designing cost effective procedures for the manufacture of scrolls. By way of example, the trend has been toward reducing time consuming machining operations, such as by seeking to reduce finish stock allowances to less than about several millimeters, while at the same time reducing section thicknesses and optimizing the material strengths. The efficient manufacture of sound castings having these desired characteristics has been difficult to achieve using existing materials and systems, especially in castings having smaller section thickness (e.g., thicknesses as low as about four millimeters), because of the resulting nonuniform formation of undesirable microstructures. Absent additional expensive, time consuming and potentially inefficient heat treatments or finishing steps (e.g., to clean up or remove undesired undercooled graphite formations in the microstructure), significant volumes of high integrity scroll castings are often not obtainable over short periods of time.
An example of a popularly employed cast iron material that has been employed for scroll compressors in relatively recent years is a gray iron having a composition as disclosed herein, but absent any high performance inoculant (as defined herein). This material, however, suffers one or more of the above discussed disadvantages, particularly the presence of undesirable amounts of undercooled structure in thin sections. Accordingly, even though sound castings are achieved, the manufacture of high integrity scrolls require expensive and substantial time consuming post-casting finishing steps. Gray iron is addressed in Metals Handbook, 9th. Ed., Vol. 15, pp 629-646, hereby expressly incorporated by reference.
Accordingly, what is needed is a system that permits casting of an intricately shaped article, particularly a scroll, and which will have a tensile strength in excess of 250 MPa, and excellent machinability and wear resistance characteristics. The system should result in an as-cast article requiring no post casting heat treatment, but still providing a substantially homogeneous microstructure having, throughout, a matrix of medium and coarse pearlite and being substantially free of steadite or having steadite present in controlled amounts. The microstructure should also include a generally uniform dispersion of relatively fine type A graphite flakes, and should be attainable regardless of section thickness (e.g., regardless of whether the section thickness exceeds common thicknesses on the order of about 30 mm or is less than about 4 mm). Any resulting undercooled structure (e.g., such as that potentially encountered at vane tips) should exhibit excellent machinability to permit rapid and easy removal of such structure while maximizing as-cast yield, and reducing post-casting finishing inefficiencies. The material should permit the efficient manufacture of scrolls having substantially thinner section thicknesses than previously. For example, high integrity as-cast section thicknesses (e.g., without limitation, for a vane) of as low as about four millimeters should be possible.
The present invention satisfies the above by providing an improved system for making a gray cast iron article, particularly a cast scroll. Other advantages and objects of the present invention will become apparent to those skilled in the art from the subsequent detailed description, the drawings and appended claims.